1. Field of the Invention
The present invention relates to water distribution systems for evaporative coolers and, more particularly, to a water distribution system for controlling distribution of water in a profile across a media to avoid dry spots, scaling, streaking and application of excess water.
2. Description of Related Art
Evaporative cooling appears to be a simple process of passing hot dry air through a wet pad or media to evaporate the water and cool the air. In reality, there are three complex mechanical and chemical processes taking place in an evaporative cooler. The first process is the air system which is controlled by the psychrometric properties of the air and the efficiency of the media. The second process is the water delivery system that has to ensure that the media has sufficient water in an effective profile for evaporation and that the media is uniformly wetted. The third process is the control of the concentration of minerals in the water where water feed and discharge rates are controlled so that the naturally occurring minerals in the water remain in solution after water evaporation and are disposed of prior to precipitating on the media. Almost all evaporative coolers built to date have made only first order approximations for one or more of the processes and have either ignored or been unaware of the others.
The air around us is essentially a constant composition of gases (nitrogen, oxygen, carbon dioxide and others) and varying amounts of water vapor. It also contains particulate impurities such as dust and organic material, which have little practical impact on the process, unless the unit is in a very dusty environment where special features are needed, therefore no discussion of particulates is included in the following discussion. The gas component of air behaves in accordance with Boyle's and Charles' laws, i.e. the volume of the gas varies inversely with the absolute pressure and directly with the absolute temperature, respectively, and the total pressure is the sum of the partial pressures. The amount of moisture in the air is dependant on the amount of moisture available and the temperature and barometric pressure of the air. This is limited to a maximum saturation value based on the air temperature and pressure and the psychrometric behavior of water vapor. As moisture is added to or removed from the air, water is either evaporated or condensed. This change in phase captures or releases energy. In evaporative cooling applications, the evaporation of water absorbs heat. The movement of the heat from the air to the water vapor happens without a change in air volume or air pressure and results in a lowering of the temperature of the air. The relationships between pressure, temperature, humidity, density and heat content are most commonly shown graphically on psychrometric charts. These relationships are very well defined and have been the subject of extensive research. Applying the psychrometric chart to the evaporative cooling process is easy for any one particular set of operating conditions. If one knows the entering air temperature (inlet dry bulb), the relative humidity of the inlet air, the barometric pressure and the volume of air being cooled one can calculate the theoretical amount of moisture that can be evaporated into the airstream and the resulting temperature reduction.
Actual operating conditions change constantly. The inlet air temperature, the relative humidity and barometric pressure are the detailed measurements of what is generally referred to as the “Weather”. Most evaporative cooler manufacturers design their equipment to handle a specific air flow rate at standard conditions and size the evaporation media for this flow rate. The efficiency of the evaporative cooler is determined by the air flow rate over the chosen media. Each media type has physical characteristics that determine how fast and thoroughly the water can be evaporated into the airstream. The most common evaporative cooling media in use today is a corrugated kraft type paper. The market leader in this type of media is Munters Corp. which markets its media under the brand names Cel Dek and Glacier-Cor. Depending on the thickness of the media used and the velocity of the air flowing through the media, the saturation effectiveness (efficiency) can range from less than 60% to about 98 or 99%.
The majority of existing evaporative coolers are controlled by an on/off switch or with a downstream thermostat which turns the evaporative coolers either on or off. The efficiency of the evaporative cooler changes with the barometric pressure, the partial pressure of the water vapor and the air temperature with the impact of these being magnified by the physical condition of the cooler. The conventional evaporative cooler does not attempt to account for or control any of these process variables to optimize efficiency and account for such variation in environmental conditions.
To obtain maximum evaporation, the media must be adequately wetted. Most conventional evaporative coolers have a large basin or sump filled with water that is pumped to a perforated header pipe at the top of the media. The water is sprayed from the header pipe up to a deflector shield and runs down onto the top of the media. Excess water is applied to ensure adequate distribution and complete saturation of the media. The water not evaporated drains into the sump to be reused. All recirculating evaporative cooler manufacturers recommend that a portion of the recirculating water be discarded and replaced with fresh water added to the sump to keep the water quality at a minimum quality level.
The media removes significant amounts of airborne contaminants from the air as it passes through the media and the return water rinses a portion of the contaminants off the media and carries them to the sump. In addition, naturally occurring salts in the water supply become concentrated on the surface of the media and are partially rinsed into the sump. While some of these contaminants and concentrated minerals are discharged in the bleed stream, a significant amount are entrained in the sump water and are recirculated back onto the media.
The pumps used in most recirculating type evaporative coolers are submersible centrifugal pumps. These inexpensive pumps are not precision pieces of equipment and wear quickly as the debris is recirculated. This deterioration of the pump leads to fairly rapid changes in the delivery head for the pump. This change in the output of the pump renders it difficult to regulate the water flow across the media. The distribution header pipe uses large holes on relatively large hole spacing to minimize debris fouling and plugging. The end result is an uneven water distribution and occasionally dry strips on the media. Constant maintenance is required to adjust and maintain an adequate supply of water for the media. These systems attempt to cure uneven water flow by pumping an excess amount of water to the media. This excess amount of water occupies space in the air flutes of the media which reduces airflow and increases the velocity in the air passage increasing the potential for water entrainment and carryover.
The most overlooked aspect of evaporative cooling is controlling the concentration of dissolved minerals as water is evaporated on the media. The water supply for evaporative coolers is typically domestic water which contains a number of compounds. Of these silicon and calcium carbonate are the more important from an evaporative cooler performance perspective. As water is evaporated by the air passing through the media, it leaves behind all of the minerals in a reduced volume of water flowing down the media. Each mineral compound has a solubility limit. That is, when the concentration of a particular compound reaches a known concentration, the compound begins to precipitate. In evaporative coolers the most predominant form of precipitate is calcium carbonate scale on the media. This hard water scale does not re-dissolve when rewetted. Once formed on the media it forms an insulating layer reducing the saturation efficiency and clogs the air and water distribution channels.
Recirculating evaporative coolers reapply the sump water to the media. Each time the water is applied, some of it evaporates and the concentration of the minerals builds up in the water. All evaporative cooler manufacturers either bleed some of the recirculating water off or dump the sump water occasionally to try to maintain an acceptable mineral concentration (called cycles of concentration in the industry). In simple terms, cycles of concentration, in a non feed and bleed situation, is the ratio of the ending volume of water to the initial volume of water. In a feed and bleed situation it is an exponential function of system volume, rate of feed and bleed, and duration of feed and bleed. For example, in a once through system where ten (10) gallons of water enters the media and nine (9) gallons are evaporated leaving one (1) gallon to exit, the media the cycles of concentration would be ten (10) divided by one (1) or ten (10) cycles of concentration. Most sumps have a float actuated make up valve to add water to the sump. This mixes the fresh water with the concentrated minerals to reduce the concentration. As a practical matter, however, some of this fresh water is also discharged such that the resulting water being distributed on the media will always have higher levels of dissolved minerals than the inlet water.
If the water distribution system allows the water in any area to become too concentrated with dissolved minerals before it leaves the media, the media will start to scale. Once scaling begins, the process threshold for additional scaling is reduced such that the salt crystals will grow whenever the water surrounding them is near the precipitation point. This occurs after scaling starts and the recirculating water must be kept at a lower dissolved solids concentration than would be allowed if the scale had not started.
To date, the best solution for improving cooler performance and control of media scaling is that of eliminating a recirculating system in favor of a single water pass system. The single pass systems provide water to the top of the media and let it flow through the media and the flow therefrom is drained. There are several challenges that must be addressed to have an effective single pass system. First, one must incorporate sensors and controls to regulate the water introduced to the media. Second, the flow volume of water must be sufficient to wet the media completely and yet the flow must be limited so as to avoid wasting large amounts of water. Some existing systems use a timer based controller to achieve the water flow control. Another type of system uses an inlet temperature sensor or a sensor within the media coupled with a timer to control the flow of water. These systems have the significant disadvantage of using too much water or from using an insufficient amount of water resulting in drying out and scaling of the media. These limitations have limited their commercial acceptability.
Various prior art evaporative cooler systems are described in the patents listed below.
U.S. Pat. No. 4,968,457 describes a non circulating control for an evaporative cooler. The water flow is metered by a simple solenoid valve which does not take into consideration change in flow rate as a function of inlet line pressure. Therefore, the amount of water delivered at different times of the day will vary with changes in domestic water line pressure. Furthermore, there is no understanding of the need for a change of water flow rates as a function of the hardness of the inlet water nor is there a discussion of providing more water than is evaporated to keep the media from scaling. A sensor for controlling operation of a solenoid valve is placed downstream of spray nozzles ejecting water to the media to sense the temperature or the humidity. There is no understanding that the cooling process is primarily dependant on the inlet air conditions.
U.S. Pat. No. 5,775,580 is directed to a non circulating evaporative cooler for primarily eliminating the dripping of water from the media. This will result in at least a part of the media becoming dry with resulting deposit of salts and compromise of the integrity of the media and its effectiveness unless pure water is used.
U.S. Pat. No. 6,367,277 discloses the use of fresh water makeup to minimize scaling in a recirculating evaporative cooler system. There is no disclosure relating to controlling the hardness of the water at the point of evaporation on the media nor does this system minimize the amount of water used. It also requires bleed of a substantial amount of the recirculating water to keep the minerals from precipitating out. No understanding of the varying conditions from location to location and the effect thereof on the efficiency of the evaporative cooler is set forth.
There are several types of problems associated with heavy scale formation on the media in an evaporative cooler where evaporative cooling occurs. First, there is a decreased air flow through the media because the air channels therewithin become more or less plugged. To maintain an adequate air volume, the velocity of the air through the media must increase. At speeds above 650 feet per minute, there is a tendency for small droplets of water to become entrained in the airstream unless other steps are taken. These droplets create corrosion and other wetting problems unacceptable to the user. Second, at localized concentrations of salts, the pH in those areas decreases dramatically. The lower pH will allow the water to leach the resin and delignify the cellulose in the media and cause premature structural failure of the media.
Indoor air quality has become a growing concern as modern office and industrial buildings become more energy efficient and better insulated. Various regulations cover how much fresh outside air must be introduced into the HVAC system in a building. This outside air is rarely at the desired temperature and relative humidity. In the southwest of the United States, the air is generally much dryer and hotter then desired. This means that the makeup air requires cooling and humidification before it can be introduced into the building. Conventional chilled water systems in large commercial buildings use large cooling towers and chillers to supply the cooling for the building. These systems are usually on or off and when on use considerable electricity to operate. Direct evaporative cooling has been used to reduce this electrical demand by evaporatively cooling the makeup air prior to use. These applications have been plagued by the same scaling and lack of control problems described above.
Evaporative cooling is often used in dusty industrial environments. Historically, recirculating evaporative coolers become plugged with dust. Often pre-filters are installed upstream of the evaporative cooler to remove the dust present in the air. Poor maintenance often resulted in filter overloading, filter failure and media plugging. One approach to this problem of dust has been that of using an excess water flow controlled by only a timer for dust control.
These results were not particularly successful. A further unit uses a fresh water makeup header to try to control the dust buildup but a timer is used to activate the flush and it has not proved to be effective.